My Drupal knowledge is well out of date, but the indisputable fact that the Drupal architecture like all software architecture imposes assumptions, many many more assumptions than a low-level framework like Rails (which tries to keep its assumptions as broadly applicable to all HTTP usage as possible). These assumptions imply tradeoffs. If you think there are no tradeoffs you need to step back and realize that Drupal has corrupted your mind and the limits of your web development ability will be constrained by whatever ideas Drupal core is able to harvest and integrate from the wider world of web development.